1. Field of the Invention
This invention relates to packaging optical fibers and more specifically to packaging a multimode laser diode that is side-coupled with a double-clad fiber.
2. Description of the Related Art
Optical fiber lasers and fiber amplifiers have become indispensable components in building optical networks for the transmission and amplification of optical signals. Optical amplification is achieved by pumping the fiber""s doped core. Typically, the pump light is coupled directly into the doped core. This requires single spatial mode diffraction limited laser diodes with an emission aperture of a few microns. This process is expensive because of the sub-micron alignment and mechanical stability required to achieve efficient and stable diode to fiber coupling. A relatively current development is the use of double-clad active fibers that consist of a single mode fiber doped core, an inner cladding and an outer cladding. The large diameter and numerical aperture of the outer cladding make it possible to efficiently couple spatially incoherent emission from high power multimode diodes into the fiber. These broad area laser diodes are approximately ten times cheaper than pigtailed single mode laser diodes.
As shown in FIG. 1a, a multimode diode laser 10 includes a chip 12 on a submount 14 that emits a laser beam 16 in a direction parallel to the diode""s mounting surface 18. As shown in FIGS. 1b and 1c, the laser diode submount 14 can be mounted on a base plate 15 with diode mounting surface 18 parallel to the base plate surface 19. In this configuration, the laser beam 16 is emitted in a direction parallel to the surfaces 18 and 19, corresponding to the Z direction as described in FIG. 1a. The laser beam 16 can be redirected by the use of a 45xc2x0 mirror 25 or any other reflective surface as shown in FIG. 1c. In this case, the laser beam 16 is now propagating in the direction Y perpendicular to the base plate surface 19. In the configuration shown in FIG. 1d, the chip 12 can be mounted on the side of a submount 14, which has a chip mounting surface 18 that is perpendicular to the base plate surface 19. The choice of diode configuration will be dictated by the application, package size limitations and type of coupler. Multimode diode laser 10 suitably comprises either a single broad emitting area or an array of single transverse mode lasers.
Because the active region 20 of chip 12 is more confined in the Y direction than in the X direction, the intensity distribution of beam 16 is more diffracted in the direction of high confinement, leading to an elliptical distribution with the slow axis 22 along the X direction and a fast axis 24 along the Y direction. When the chip 12 is mounted parallel to the base plate surface 19 as shown in FIG. 1b, fast axis 24 is perpendicular to base plate surface 19 and active region 20. When mirror 25 is used to redirect the beam as shown in FIG. 1c, or when the laser chip is mounted on the end of submount 14 as shown in FIG. 1d, fast axis 24 is parallel to base plate surface 19.
Because the thickness of the semiconductor active region 20 is much smaller than its width, focusing along the fast axis is easier than along the slow axis. A number of different optical systems have been proposed that use some combination of one or more simple lenses, cylindrical lenses, microlens arrays, fibers and waveguides to reimage the sources so as to reduce one of the asymmetries inherent in laser bars or multimode laser diodes with broad area emitters. For example, the use of lenses is disclosed in U.S. Pat. No. 4,428,647 and the use of fibers in U.S. Pat. No. 4,763,975.
The multimode pump can be coupled into the fiber""s inner cladding in a number of ways. The multimode pump shown in FIG. 1b can be xe2x80x9cbuttxe2x80x9d coupled to an open end of the fiber. The orientation of the fast-axis is not critical because the open end of the fiber is axially symmetric. However, this configuration does not provide access to both ends of the fiber. The same multimode pump can also be coupled to the fiber by using a special fused fiber coupler which allows pump light to be transferred from a multimode fiber into the fiber""s inner cladding without disturbing the signal propagating in the core. However, such fused fiber couplers are inefficient, complex and expensive.
U.S. Pat. No. 5,854,865 to Goldberg teaches forming a groove in the side of the double-clad fiber to xe2x80x9cside-couplexe2x80x9d the multimode pump into the fiber. The laser diode is placed on the other side of the fiber so that the pump propagates laterally through the fiber and impinges on the side of the groove, which specularly reflects the pump along the horizontal axis of the fiber confined in the inner cladding. The formation of such grooves is a complex manufacturing process that weakens the fibers and requires relative large inner cladding diameters, which reduces coupling efficiency.
Packaging the multimode diodes, couplers and fibers is oftentimes the most challenging aspect of providing a commercially viable fiber laser or fiber amplifier. The package must provide (a) sufficient and cost-effective heat sinking, (b) efficiently couple the pump into the fiber, which means, in part, orienting the fast-axis parallel to the fiber at the coupler when the coupling aperture is not axially symmetric, (c) thermal isolation of the fiber from the diode, and (d) a simple, compact and reliable package.
International publication WO 01/48878A1 entitled xe2x80x9cHybrid Optical Coupling Componentxe2x80x9d discloses a configuration for packaging a multimode diode laser of the type shown in FIG. 1d above with Goldberg""s V-groove fiber coupler. A fiber 30 formed with the v-groove coupler 32 is mounted directly above diode 10 in a glass block 31 so that beam 16 emits perpendicular to the plane of the mounting surface with its fast-axis 24 oriented parallel to fiber 30. The coupling aperture into the fiber is not axially symmetric so proper orientation of the fast-axis is critical to efficient coupling. Beam 16 passes through imaging lens 34, propagates laterally through fiber 30 and is reflected off the v-groove horizontally along the fiber. This package provides adequate but sub-optimal heat sinking. The chip 12 is mounted flat on end surface 18 of sub-mount 14 to maximize surface area but the heat path from the chip 12 to end surface 18 of sub-mount 14, and to surface 19 of base plate 15 is not straight. This makes the heat path longer, reducing the efficiency of the heat sinking. Furthermore, The package provides poor thermal isolation because fiber 30 is mounted directly on top of diode 10. Heat and particularly fluctuations in temperature can change the characteristics of the gain fiber. In addition, placement of fiber 30 on top of the diode is a delicate and unreliable packaging challenge and also increases the package height, which is a critical parameter in many applications.
The present invention provides a simple, compact and reliable package for optical amplification that provides efficient coupling of the multimode pump to the double-clad fiber, sufficient heat sinking and good thermal isolation.
This is accomplished with a multimode laser diode mounted with its active region parallel to a mounting surface. The diode emits a beam parallel to the mounting surface with a fast-axis of the beam perpendicular to the diode""s active region and the mounting surface. A beam rotator rotates the beam so that the beam""s fast-axis is oriented parallel to the mounting surface. A multimode diode package of only the diode and rotator may be provided for integration with a side-coupled fiber.
The beam rotator comprises a pair of reflective surfaces such as provided by mirrors, a prism or a tilted v-shaped mirror that perform a pair of xe2x80x9coff-plane bouncesxe2x80x9d to rotate the beam""s fast axis. In an XYZ coordinate system, if the diode lies in the XZ-plane and emits the beam along the Z-axis with its fast-axis along the Y-axis, the first reflective surface folds the beam along the Y-axis and its fast-axis along the Z-axis. The second reflective surface folds the beam along the X-axis and leaves the fast-axis along the Z-axis. The fiber is mounted in the XZ-plane along the Z-axis parallel to the fast-axis.
A pump-coupler package will also include a double-clad fiber having a core, an inner cladding and an outer cladding is mounted on the mounting surface adjacent the laser diode and substantially parallel to the orientation of the beam""s fast axis. An imaging lens is placed between the rotator and a side-coupler that couples the shaped and properly oriented beam into the fiber""s inner cladding through a side of the fiber. In turn, the beam is coupled into an active portion of the double-clad fiber having a doped core to provide optical amplification. Suitable side-couplers include a V-groove or a TIR coupler, which can be formed directly on the active double-clad fiber or on a passive double-clad fiber connected to the active-double clad fiber. The diode laser and rotator can be integrated in a single package with the other components or provided as a package for integration into a second package.